1. Field of the Invention
This invention relates to a cast light metal piston consisting of an aluminium alloy or a magnesium alloy particularly an aluminium alloy of type AlSi12 CuNiMg, for internal combustion engines, in which piston an embedded, insert comprising a ceramic material having a low thermal conductivity of 0.1 to 5 W/mK, a low coefficient of expansion of 0 to 5.times.10.sup.-6 /K and a low modulus of elasticity of 1 to 5.times.10.sup.4 N/mm.sup.2 is provided in those zones of the piston head which are highly thermally stressed and/or for heat insulation as the trench of pistons of precombustion chamber engines or the rim of the combustion chamber recess of pistons of direct injection engines.
2. Discussion of Prior Art
In internal combustion engines the increasing requirements for a higher power per cylinder, for higher exhaust gas temperatures, for a limitation or decrease of the dissipation of heat through the cooling system and for a higher utilization of the energy content of the fuel by secondary measures, such as the use of exhaust gas turbochargers, involve high thermal stresses on the pistons and require a heat insulation of the pistons
It has recently been attempted to avoid or at least reduce such stresses by the provision of protecting ceramic elements in the thermally stressed zones of light metal pistons of internal combustion engines. For instance, J. H. Stang in Designing Adiabatic Engine Components, Ser. No. 780,069, has proposed to provide a light metal piston with a screwed-on ceramic head, which is insulated from the base by steel discs. The head of such light metal piston may assume a temperature up to 900.degree. C. Disadvantages reside in the manufacturing costs of a protecting ceramic element which has the required strength and in the relatively large dead space above the first ring groove.
The heat of another known light metal piston is formed with a combustion chamber recess and contains at the rim of said recess an embedded aluminum titanate ring, which has an adequate compressive strength. Aluminum titanate (Al.sub.2 TiO.sub.5) is a ceramic material which has a low thermal conductivity (.lambda.=2 W/mK), a low coefficient of expansion (.alpha.=1.5.times.10.sup.-6 /K), a low modulus of elasticity (E=2.times.10.sup.4 N/mm.sup.2), a very high fatigue strength and a high thermal shock resistance. After the casting operation the solidifying light metal contracts so as to form a shrinkage joint between the ceramic ring and the surrounding light metal. The resulting compressive stresses ensure the fixation of the ring and give rise to compressive stresses in the ring.
Because the light metal piston is deformed during the operation of the engine, the piston will be subjected adjacent at its head to tensile and compressive stresses and the resulting tensile stresses in the ring will be superimposed on the compressive stresses which are due to the shrinkage fit. As long as the compressive stresses in the ring exceed the tensile stresses, there will be no or only low tensile stresses in the ring itself. That condition is decisive for the service life of the ring because ceramic materials as a rule will withstand much higher compressive stresses than tensile stresses without failure.
For instance, the highest permissible tensile stresses for aluminum titanate are below 40 N/mm.sup.2 whereas the material will not fail under compressive stresses up to ten times that value. For this reason a fracture or loosening of the ring will be avoided with adequate safety as long as the compressive stresses are sufficiently high relative to the tensile stresses. But this is not ensured during a prolonged operation of the piston because the compressive stresses which are due to the shrinkage fit will decrease after a relatively short operating time of the piston as a result of creep and/or relaxation phenomena, which are due to the temperatures in excess of 250.degree. C. arising in the light metal piston. The function of the light metal piston will be adversely affected by the resulting loosening or fracture of the ring and consequential damage must be expected.
For instance, for a cast light metal piston comprising an aluminum alloy of type AlSi12CuNiMg and comprising an aluminum titanate ring embedded at the rim of the combustion chamber recess it has been found that the initial compressive stress of about 60 N/mm.sup.2 which was due to the shrinkage joint had decreased to 20 N/mm.sup.2 after an operation for 100 hours owing to the constant elongation occurring at a temperature of 250.degree. C. The creep and/or relaxation of the aluminum alloy give rise to high tensile stresses in the ring so that cracks are formed in the ring or the ring comes loose. This adversely affects the function of the piston. For this reason it has not been usual thus far to provide light metal pistons of internal combustion engines with embedded ceramic inserts in thermally highly stressed zones and/or for heat insulation of the piston head as the trench of pistons of precombustion chamber engines or the rim of the combustion chamber recess of pistons of direct injecting engines.